Lay two squares of 3/4-inch plywood measuring 20 by 20 inches (the ends of the bench) flat on a work surface, side by side. Measure up 3 inches from the bottom of both squares and rule a line across.
Set the blade on your dado saw to cut a dado (a groove or slot) that is 3/4-inch wide and 1/4-inch deep. Cut a dado in each of the bench ends on the line you marked.
Measure in 3 inches from the sides of each end piece, under the dado groove. Scribe an arc that goes from each 3-inch mark and is 2 inches high at the midpoint. Cut the arc out of the plywood with a scroll saw or jigsaw. These are the feet of the bench. You can cut out a more decorative design than just an arc, if desired.
Slot a piece of 20-by-31-inch, 3/4-inch-thick plywood in between the dado grooves on the bench ends. This is the base of the bench, and will be 3 inches off the ground. Glue the base in place, checking with a carpenter's square that the base is at true right angles to the bench ends. Leave to dry for an hour or so, depending on the drying time of your wood glue.
Turn the bench ends upside down, so the underside of the base is exposed. Position an angle bracket between the bench ends and the base, around an inch in from the sides of the bench ends. Secure the angle brackets to the bench ends and the underside of the base with 1/2-inch wood screws driven through the screw holes in the brackets. This will give strength and add support to the bench base. Turn the ends and base back upright.
Measure in 3 inches from either side of two planks of 20-by-32-inch, 3/4-inch plywood (the bench back and front) and scribe an arc that goes between each 3-inch mark and peaks at 2 inches high. Cut out the arc with a scroll or jigsaw, like you did for the bench ends.
Place the front and back panels over the sides of the bench, with the arc side down. They will form corners with the ends of the bench ends, and be flush with the top of the bench ends as well. Glue the front and back panels in place, then secure with 1-inch wood screws through the sides of the front and back panels into the abutting edges of the end panels. You will now have a rectangular box.
Center a 29-inch-long piano hinge along one 32-inch side of a sheet of 21 1/2-inch-by-32-inch sheet of plywood, which is the deacon bench lid. Secure the hinge in place with 1/2-inch wood screws driven through the screw holes in the hinge.
Center the lid over the top of the box, and secure the remaining half of the piano hinge along the back panel with 1/2-inch wood screws driven through the screw holes of the hinge. Finish the deacon's bench in the desired wood finish, and pop a screw cap over the visible screw holes.